1. Field of the Invention
The present invention relates to an image processor and an image processing method. More particularly, the present invention relates to an image processor configured to decrease color unevenness caused by individual variations in the ejection characteristics of a plurality of nozzles that eject ink.
2. Description of the Related Art
In print heads used in inkjet printing apparatus, individual variations are sometimes exhibited in the ejection characteristics (i.e., the ejection volume and ejection direction, for example) of a plurality of nozzles, due to manufacturing errors and other factors. When such variation exists, printed images become more susceptible to density unevenness.
In the related art, one established process for decreasing such density unevenness involves using head shading technology such as that disclosed in Japanese Patent Laid-Open No. H10-013674 (1998). Head shading is a technology that corrects image data according to information regarding the ejection characteristics of individual nozzles. By means of such correction, the number of ink dots that are ultimately printed is increased or decreased for each nozzle, and the concentration (density) in the printed image can be made nearly uniform across the nozzles.
However, when head shading technology as described above is used, if two or more inks are combined for color reproduction, then a phenomenon will occur wherein the coloring in the areas printed with the nozzles having non-standard ejection volumes will differ from the colors that should be printed. In other words, the above might cause inaccurate color, herein referred to as color deviation.
By way of example, consider the case of printing a blue image using nozzles exhibiting standard ejection volumes for cyan ink, and greater-than-standard ejection volumes for magenta ink. In this case, the magenta ink with the greater-than-standard ejection volumes will form dots on the print medium that are larger than the cyan dots. If such a print head is corrected by means of head shading (i.e., an HS process), then magenta will be printed using fewer dots than the standard number of dots. In other words, the number of magenta dots will be less than the number of cyan dots. As a result, the blue image regions will contain a mixture of solid cyan dots of standard size, as well as overlapping dots wherein cyan dots are printed inside larger magenta dots. The coloring in such regions will be different from the coloring in a blue image printed using cyan dots and magenta dots of standard size and number. This occurs because the ratio of the print medium occupied by solid cyan, the ratio occupied by solid magenta, and the ratio occupied by blue resulting from overlapping cyan and magenta, all differ between the above two images.
Such variation in the surface area ratios occupied by respective colors occurs not only because of variation in ejection volume, but also because of variation in ejection direction. In other words, even if density unevenness in solid cyan images or solid magenta images is resolved by the head shading of the related art, the variation in ejection characteristics will still lead to color deviation in blue images expressed by overlapping combinations of these colors. Furthermore, since the degree of color deviation differs among the regions printed by nozzles with different ejection characteristics, different shades of color are perceived in individual regions that should have the same coloring, which is noticed as color unevenness.
In addition, the ejection characteristics of the individual nozzles themselves change according to the frequency and cumulative number of ejections. This tendency is particularly large in Bubble Jet (registered trademark) print heads, wherein film boiling in the ink is produced as ejection energy, with the expansion pressure that accompanies the vaporization being utilized to eject ink. Thus, the tendency towards the color unevenness described above changes according to the frequency and cumulative number of ejections for each individual nozzle.